It has become common practice to pump a viscous fluid at high pressures down into a wellbore to crack the formation and force fracturing fluid into created cracks in order to enhance or increase the production of oil and gas hydrocarbons from wells bored into subterranean formations. The fracturing fluid is also commonly used to carry sand and other types of particles, called proppants, to hold the cracks open when the pressure is relieved. The cracks, held open by the proppants, provide additional paths for the oil or gas to reach the wellbore, which increases production from the well. This process is commonly called hydraulic fracturing or “fracking”.
A hydration unit is generally used for the hydration of fracturing fluids or hydrated fluids originating from a very viscous fracturing fluid slurry concentrate (gel) that is mixed with water in preparation for transfer to a blender unit prior to being pumped under pressure down-hole. The fracturing fluid slurry concentrate (gel) is used in a continuous hydration process in a hydration unit so as to produce hydrated fluid as needed or “on-the-fly” for the hydraulic fracturing process. Typically a gel may comprise a polymer slurry wherein a hydratable polymer is dispersed in a hydrophobic solvent (herein after referred to as an “oil based fluid”) in combination with a suspension agent and a surfactant with or without other optional additives commonly employed in well treatment applications. Because of the inherent dispersion of the hydratable polymer in an oil based fluid (i.e., the lack of affinity for each other), such a polymer slurry or polymer phase gel tends to not lump or hydrate prematurely prior to dispersion, injection or being added into water. However, the rate of polymer hydration within the gel is a critical factor particularly in continuous mix or hydration unit applications wherein the necessary hydration and associated viscosity rise must take place over a relatively short time span that corresponds to a minimum residence time of the fluids within a hydration unit during the continuous mix procedure.
In such applications, hydration is the process by which a hydratable polymer absorbs water. When the polymer is dispersed in water, its ability to absorb water dictates hydration or its hydration rate. There are several factors that determine how readily a polymer will hydrate or develop viscosity. Such factors include the pH of the system, the amount of mechanical shear applied in the initial mixing phase, the concentration of salts and the concentration of the polymer. The hydration rate can be influenced through pH control agents, which may be blended with the polymer in the gel or added to an aqueous medium. The hydration rate can also be controlled by the level of applied shear, wherein the gel-water solution's viscosity increases faster when the hydratable polymer is subjected to high amounts of shear. Fluid viscosity increases may also be influenced (particularly in low shear applications) by the salts present in the solution. The higher the salt content in the solution, the more retarded the hydration process. The extent of viscosity retardation is dependant on the concentration and the type of salt. Finally, the viscosity level achieved at a particular point in time is a function of the overall hydratable polymer concentration.
Various natural hydratable polymers are used in a polymer phase gel. In particular, modified guar works very well and develops viscosity in all electrolyte or salt bearing systems which contain such salts as KCl, NaCl, and CaCl2 concentrations. Guar gum hydrates and develops viscosity very efficiently in a pH range of 7-8 yielding viscosities of 32 to 36 cps in 2% solution of KCl. Hydroxypropyl guar (HPG) hydrates well in many salt systems at 80° F. and also develops excellent viscosity at temperatures around 40° F. Carboxymethyl hydroxypropyl guar (CMHPG) hydrates in most electrolyte make-up solutions, however, it's more sensitive to such salted electrolyte solutions than unmodified guar and HPG. CMHPG hydrates well in both cold and warm water.
In contrast to the above natural polymers, synthetic polymers may also be dispersed and hydrated, however they may not be as sensitive to pH effects. Consequently hydration and dispersion of such synthetic polymers will mainly rely more on the mixing shear applied to the aqueous medium in a hydration unit.
Generally, prior hydration units that accept a polymer phase gel and water mixture so as to produce a hydrated fluid as part of a continuous preparation of fracturing fluids have focused primarily on mechanical mechanism movement or paddle based mixing processes within a hydration unit. The paddle based mixing process requires a large mechanical paddle or beater structure that is rotatably mounted within a hydration unit. The paddle structure is mechanically rotated on bearings and driven via, for example, a chain or shaft drive, which is mechanically attached and driven by a hydraulic, electric or combustion powered drive train and/or transmission. Mechanical failure of any part of the drive train, chain links and/or bearings can shut down the hydration unit, which is expensive and time consuming to repair. Furthermore, significant torque and horse power is required to rotate the mechanical paddles at the speeds necessary for producing shear forces that increase the hydration rate of the hydratable polymer and establish the needed hydrated fluid viscosity at the hydration unit output by such a mechanical paddle or beater based system.
What is needed is a hydration unit that can provide suitable amounts of shear on a polymer phase gel and water mixture in order to sufficiently increase the hydration rate of the mixture during its residence time within the hydration unit. Furthermore, what is needed is a hydration unit that requires fewer moving parts such as paddles, bearings, chains and the like that are subject to wear and breakage resulting in extended down time to repair the hydration unit.